Pyrite and Marcasite

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Pyrite and marcasite (FeS2) are iron disulfide compounds. They look similar but have different crystal forms. Both are brittle, hard, brassy yellow with metallic luster, and opaque. Their crystal shape is the most distinguishing feature. The pyrite crystals are cubes, but the marcasite crystals are blade- or needle-shaped.

Pyrite and marcasite have been mistaken for gold because they are yellow and metallic. They are commonly known as "fool's gold." These rocks are much harder than gold, tarnish, and leave a dark streak; gold is soft, very heavy, does not tarnish, and leaves a yellow streak. Gold is malleable, but pyrite or marcasite is reduced to powder if pounded and gives off a noticeable odor of sulfur dioxide gas if heated or cooked.

Both pyrite and marcasite are common as surface coatings, veins, and concretions in coal and in dark shales associated with coal. They are referred to as "coal brasses" or "sulfur" when found as impurities in coal. Coal brasses recovered from Illinois coal have been used in the manufacture of sulfuric acid for industrial use.